1. Field of the Invention
The present invention relates to a method of inspecting a printed circuit board, a method of manufacturing a printed circuit board and an inspection device of a printed circuit board.
2. Description of the Background Art
Electronic apparatuses such as hard disk drives or cellular telephones are provided with printed circuit boards having conductor traces formed thereon. Sometimes, automatic appearance inspection such as AVI (Automatic Visual Inspection) is performed in manufacture of such printed circuit boards.
In the AVI, the presence/absence of defective pieces having defects in conductor traces, dimensional errors of conductor traces, defects in terminals and damaged resist surfaces and so on can be determined in a short period of time in the final stage of manufacturing steps of the printed circuit boards.
The AVI is performed in the following manner, for example. First, a good printed circuit board with no defect is imaged by an imaging device, and obtained image data is stored in a memory as master data. Then, a printed circuit board to be inspected is imaged by the imaging device, and obtained image data is stored in the memory as inspection target data. The master data and the inspection target data are subsequently compared with each other, so that determination as to whether or not the printed circuit board is defective is made.
In the AVI, since the determination as to whether or not the printed circuit board is defective is made based on the image data, the printed circuit board must be reliably fixed when being imaged. Therefore, an AVI device is provided with an inspection platform including a holding mechanism that holds the printed circuit board. Examples of the holding mechanism include a clamp mechanism, a vacuum suction mechanism and an electrostatic adsorption mechanism.
In an inspection platform including the clamp mechanism (hereinafter referred to as a clamp inspection platform), ends of the printed circuit board are fixed on the inspection platform by a plurality of clamps while the printed circuit board is placed on the inspection platform.
An inspection platform including the vacuum suction mechanism (hereinafter referred to as a vacuum inspection platform) has a through hole that penetrates the inspection platform from its upper surface to its lower surface, for example. In addition, a suction device that sucks in an atmosphere above the upper surface of the inspection platform through the through hole is provided on a lower surface side of the inspection platform. The suction device is operated while the printed circuit board is placed on the inspection platform, so that the printed circuit board is attracted through the through hole by the suction device to be fixed on the inspection platform.
An inspection platform including the electrostatic adsorption mechanism (hereinafter referred to as an electrostatic inspection platform) includes an insulator that forms an upper surface of the inspection platform, for example. An electrical conductor is provided below the insulator. The electrical conductor is composed of a positive electrode plate and a negative electrode plate each arranged in parallel with the upper surface of the inspection platform. Voltage is applied between the positive electrode plate and the negative electrode plate of the electrical conductor while the printed circuit board is placed on the inspection platform, so that the insulator polarizes to induce positive charges and negative charges on the upper surface of the inspection platform. Thus, an electrostatic force (Coulomb force) arising from the induced positive charges and negative charges causes the printed circuit board to be adsorbed on the upper surface of the inspection platform to be fixed on the inspection platform.
Since the ends of the printed circuit board are fixed by the plurality of clamps in the above-described clamp inspection platform, a tensile force is exerted between the plurality of ends of the printed circuit board. Therefore, the printed circuit board may be fixed on the inspection platform while having wrinkles generated thereon. In this case, it is difficult to perform accurate AVI.
If a number of holes are formed in the printed circuit board, the printed circuit board cannot be adsorbed on the inspection platform in some cases in the vacuum inspection platform. In addition, a suction force is locally generated in the portion where the through hole is formed in the inspection platform. Therefore, an uneven suction force is exerted on the printed circuit board, thereby causing strain in the printed circuit board in some cases. Also in this case, it is difficult to perform accurate AVI.
In the electrostatic inspection platform, since the object is adsorbed by the electrostatic force, a sufficient adsorption force cannot be obtained in a case where the printed circuit board and the upper surface of the inspection platform are not in close proximity to each other. Therefore, the printed circuit board cannot be adsorbed on the upper surface of the inspection platform when strain such as warp occurs in the printed circuit board. In this case, it is difficult to perform accurate AVI.
JP 2002-76569 A describes an inspection device of a printed board using the electrostatic inspection platform. In the inspection device, the printed board is pressed against the upper surface of the inspection platform by a squeeze roller while being electrostatically adsorbed on the inspection platform. This causes the printed board to be flattened on the inspection platform.
In fact, however, a frictional force or the like occurs between the printed board and the squeeze roller, causing wrinkles in the printed board in some cases. In addition, the surface of the squeeze roller is adhesive. Therefore, when the printed board happens to stick to the squeeze roller, it may be raised from the inspection platform as the squeeze roller moves. Also in these cases, it is difficult to perform accurate AVI.